Purification of perfluorocompounds



Patented Oct. 5, 1954 STATES PATENT OFFICE UNITED Phillips Petroleum Company, a corporation of Delaware N Drawing. Application September 12, 1951, Serial No. 246,337

3 Claims.

1 This invention relates to a process for the substantial removal of impurities boiling in the same boiling range as perfluorocompounds from said 2 extensive use for the catalyzed substitution of fluorine for chlorine; however, its use is limited because completely fluorinated compounds cannot perfluorocompounds. In one of its aspects this be produced by this process. In practice, antiinvention relates to a process for the substantial 5 mony pentafluoride is used for the fluorination of removal of incompletely fluorinated compounds methyl carbon atoms attached to cyclic structures containing hydrogen or chlorine atoms from the or a portion of the carbon atoms in an aliphatic completely substituted perfluorocompounds which chlorinated hydrocarbon and either vapor-phase may be, among others, non-acidic and non-basic fluorination or catalytic fluorination with cobalt perfluorocarbon, perfluoroethers, perfluorokel0 trifluoride is used to complete the fluorination tones and the like. process by replacing the residual hydrogen and In the manufacture of perfluorocompounds, all chlorine atoms. This combination process also the hydrogen atoms attached to carbon atoms of requires less elemental fluorine, which is diflicult an organic compound, except those hydrogen and expensive to produce on a large scale, than atoms Whose substitution would affect the nature direct fluorination using cobalt trifluoride alone. of the functional groups present, are replaced by unfortunately, the perfluorocompounds obtained fluorine atoms. The substitution of fluorine for in this process often still contain small quantities hydrogen in a hydrocarbon is a highly exothermic of incompletely-fluorinated compounds, that is reaction because of th high heat of formation of chlorocompounds, in addition to other undesirhydrogen-to-fluorine and carbon-to-fluorine 2O able compounds which are not readily separated bonds and, therefore, the reaction is diflicult to from the perfluorocompounds by the usual puricontrol and undesirable compounds such as inflcation methods. completely fluorinated hydrocarbons are formed. Perfluorocompounds are usually characterized In the most widely used process for the producby their high stability and lack of chemical action of completely substituted perfluorocarbons tivity. The lower-boiling compounds have low by the replacement of hydrogen atoms, the fluoritoxicity and non-flammability and are usually nation of the hydrocarbon is accomplished by diused as refrigerants and as heat transfer merect fluorination in the presence of cobalt tridiums, whereas the higher-boiling perfluorocomfluoride or. some other high-valence metal fluopounds have viscosity and density characteristics ride. The cobalt trifluoride is converted to the which permit their use as lubricants. In general, difluoride, which is then reconverted to the trithe most important property in the usefulness of fluoride with fluorine gas. The fluorination of perfluorocompounds is their chemical inertness hydrocarbons may also be performed in the vapor which permits their use in contact with strong phase by reaction of the fluorine, which is diluted chemical reagents and solvents. Unfortunately, with nitrogen, with the hydrocarbon and the high the presence of small quantities of impurities, heat of reaction dissipated by passage of the reparticularly compounds in which some of the hyaction products through copper gauze. The undrogen and chlorine atoms have not been replaced reacted hydrocarbons are then separated from by fluorine atoms, destroys this property of the perfluorocarbons by distillation; however, chemical inertness and results in instability of some of the incompletely substituted perfluorothe compound. carbons and other undesirable compounds are According to this invention there is provided a also found in the finished product since these process for the purification of perfluorocomcompounds boil in the same boiling range of the pounds whereby undesirable compounds, boiling perfluorocarbons. Another method of fluorinain the same boilin range as t e D fi 0 0mtion of organic compounds comprises the electrolpound to be purified and comprising incompletely ysis of a solution of the organic material in the fluorinated compou ds Conta n g hydrogen or presence of hydrogen fluoride or some other comchlorine atoms, are eifectively removed by reacpound which will permit the solution to conduct tion of said undesirable compounds with a nitroelectricity. gen base, such as ammonia or an amine, at either An important method of fluorination is the atmospheric conditions or temperatures within process in which the fluorine atoms are exchanged the range of 50 to 250 (3., preferably 25 C. or for other halogens, usually chlorine, which have lower, and pressures sufiicient to maintain the previously been substituted for the hydrogen liquid phase and then by separation of the solid atoms of the organic compound. Antimony penreaction product formed from the perfluorocom tafluoride is avery reactive reagent and has found pound by filtration, adsorption on an adsorbing J agent or by vacuum distillation of the purified material from the dissolved impurity complex or compound.

Although I believe the impurities removed according to this invention to be incompletely fluorinated compound containing some molecules in which all the hydrogen or chlorine atoms have not been replaced by fluorine atoms, and I have discussed them as such, other impurities of an unknown origin, and which are effectively removed by the process of my invention, may be present.

As stated, the process of my invention is concerned with the conversion of the impurity contained in the perfluorocompound into a solid product which may be readily removed from the purified perfiuorocompound by the usual methods of separation such as adsorption on an adsorption agent or by filtration. The formation of this solid product is obtained by the agitation of either a nitrogen base or a mixture of nitrogen bases, such as ammonia or amines, with the contaminated perfiuorocompound, preferably Within the temperature range of 50 to 250 C., for a period of time sufficient for the substantial completion of the reaction. Although I prefer to carry-out the process of my invention in the temperature range of 50 to 250 C. and I usually maintain a temperature of around C. or lower, I may carry out the reaction at any suitable temperature at which both the perfluorocompound and the nitrogen base are in the liquid phase. I may also operate at either atmospheric pressure or some other pressure which will enable me to perform the reaction in the liquid phase and at a more suitable temperature. The reaction time necessary for the substantial completion of the reaction cannot be readily specified since the various nitrogen bases and perfluorocompounds impurities do not have the same reaction rates. The reaction time and other process variables can, however, be quickly determined by those skilled in the art by mere routine test with the contaminated perfiuorocompound and the nitrogen base, since the reaction product is usually a precipitate. This spot check also enables one skilled in the art to determine whether or not a perfiuorocompound of questionable contamination must be purified.

Example I A mixture of 50 cc. of perfluorodimethylcyclohexane and 50 cc. of di-n-propyiamine were agitated for minutes at room temperature and atmospheric pressure with a noticeable evolution of heat resulting from the high heat of reaction. The solid compound formed was first removed by filtration and the residual material by a silica gel adsorbent to substantially improve the purity of the perfiuorodimethylcyclohexane as shown by the increase in density from 1.8469 to 1.8486 and the narrowing of the boiling range from 191 to 216 F. for the untreated perfiuorocompound to 205 to 216 F. for the treated compound.

Eatample II A mixture of 10 cc. of perfiuorodimethylcyclohexane and 16.8 grams of anhydrous ammonia were agitated for 2 hours at approximately 35 C. and atmospheric pressure. After removal of the solid reaction product containing the impurities the density of the treated perfiuorodimethylcyclohexane was found to have increased from 1.8469 to 1.8491. In'order to determine the effectiveness of the purification step, the previously treated perfiuorodimethylcyclohexane was again reacted with anhydrous ammonia for a period of approximately one week. The density of the recovered product had been increased to only 1.8492, thereby showing that the removal of the impurities is substantially complete.

It is preferred to utilize ammonia and primary and secondary alkyl, alkenyl and aryl amines in the purification process described in this specification. Tertiary amines such as tributyl amine and pyridine may be used, but they react relatively slowly and are therefore less desirable. Primary and secondary amines containing methyl, ethyl, propyl, butyl, amyl, hexyl, phenyl, benzyl and similar groups are satisfactorily utilized. In the case of secondary amines, the groups need not be alike.

Herein and in the claims the term perfiuorocompound is intended to include only completely fluorinated compounds, that is compounds containing fluorine in the place of all hydrogens attached to carbon atoms, as defined above.

Reasonable variation and modification are possible within the scope of the foregoing disclosure and the appended claims to the invention the essence of which is that a method has been set forth for the purification of perfluorocompounds by reaction with a nitrogen-containing base such as ammonia or an amine to form a separable product with any impurities present.

I claim:

1. The purification of perfluorodimethylcyclohexane which comprises agitating the said compound with di-normal-propylamine to form a reaction product by interaction of the said di-normal-propylamine with impurities contained in said perfluorodimethylcyclohexane and then removing the said reaction product from the said perfiuorodimethylcyclohexane.

2. A method according to claim 1 wherein the said reaction product is removed by filtration followed by silica gel adsorption.

3. The purification of perfluorodimethylcyclohexane to remove therefrom incompletely fluorinated compounds which comprises intimately contacting the same with approximately an equivalent amount of di-n-propylamine for about 1 hour thus forming a solid compound and then removing the said solid compound from the perfiuorodimethylcyclohexane by filtration and then passing said perfluorodimethylcyclohexane through an adsorbent to further improve its purity.

References Cited in the file of this patent UNITED STATES PATENTS Number 

4. THE PURIFICATION OF PERFLUORODIMETHYLCYCLOHEXANE WHICH COMPRISES AGITATING THE SAID COMPOUND WITH DI-NORMAL-PROPYLAMINE TO FORM A REACTION PRODUCT BY INTERACTION OF THE SAID DI-NORMAL-PROPYLAMINE WITH IMPURITIES CONTAININE IN SAID PERFLUORODIMETHYLCYCLOHEXANE AND THAN REMOVING THE SAID REACTION PRODUCT FROM THE SAID PERFLUORODIMETHYLCYCLOHEXANE. 